The present invention relates in general to signal adapting circuitry, and, in particular, to a new and useful automatic line build out circuit for use in processing digital data coming to a receiver over different cables having various lengths.
In a digital communications system, digital data is sent and received along several cables of varying lengths. The digital data becomes much weaker and more distorted when traveling through a long cable path as opposed to a short one. For example, the data is weaker and more distorted when traveling along a 2 kilometer cable length than a 1 meter cable length.
The data sent through long cable lengths must be amplified and equalized to try and reconstruct the "clean" data pulses originally transmitted. The data signal pulses that go through the shortest cables require almost no amplification or equalization. Amplification involves increasing the height of the signal's waveform peak. Equalization involves undistorting the shape of the signal's waveform so it looks like the signal that was first transmitted through the cable. This is done by increasing the amplitude of the signal wave in proportion to attenuation, or drop in wave amplitude, caused by the length of the communications cable.
Because signals traveling through several different lengths of cables are often transmitted to a single receiver, the degree of attenuation of the signals at the receiver is also varied. Rather than have a separate receiver for each length of cable to receive each differently attenuated signal, the state of the art is to provide some type of circuitry before the receiver to make each signal conform to standard characteristics.
The receiver must be able to accommodate the most attenuated signals from the longest length of cable. Therefore, the standard characteristics that signal waves must conform to before being input to the receiver are those resulting from their travelling through the longest cable length in the communication system.
Each data pulse train making up a communication signal is actually composed of two or more pulse trains of varying frequency. The lowest frequency pulse train is called the fundamental. The higher frequency pulse trains are whole number multiples of the fundamental and are called harmonics. FIG. 1 shows a fundamental and a harmonic that combine to form a data pulse train.
When a communication signal travels through a long cable, the higher frequency harmonics are attenuated more than the lower ones, distorting the overall signal. These upper harmonics must be amplified more than the lower ones on the receiving end of the cable in order to reconstruct the signal that was originally transmitted. For this reason, the second block of a receiver, called an equalization amplifier, performs this function in known communication systems.
Equalization amplifiers are not necessarily variable, meaning that they can not adjust themselves to a varying degree of attenuation for every signal which travels through a different length of cable. For simplicity equalization amplifiers are set up to handle the worse case of attenuation or the signals from the longest length of cable. The signals from the smaller lengths must then be distorted to the degree of the longest cable in order to be input to the single equalization amplifier. If this did not happen, the equalization amplifier would over amplify the signals from the shorter cable lengths, thereby distorting these clearer signals. What actually happens then is that the clearer signals from the shorter cables must actually be attenuated or degraded before they are input to the equalization amplifier or receiver.
Therefore, known communications systems provide a circuit which is designed to attenuate the signals from different cable lengths to conform to the degree of attenuation seen in a signal from the longest cable length. These known circuits are called Automatic Line Build Out circuits or ALBOs.
Prior art ALBOs use voltage gain techniques to provide the attenuation needed by the stronger signals from the shorter cable lengths. These ALBOs use devices such as field effect transistors (FET) to attenuate the amplitude of the incoming signal wave (See FIG. 2). In FIG. 2, Vc, a signal corresponding in proportion to the cable length and generated by a controlling device, is input to the FET, as shown. This Vc adjusts the resistance of the FET to decrease the amplitude of the signal wave a certain degree, corresponding to the cable length. The signal is then applied to the equalization amplifiers (EQ amp.). This will make all cable lengths attentuate to he degree of signals from the longest cable length (see FIG. 3).
The problem with the prior art ALBOs is that they only adjust signals in terms of amplitude or output voltage and not distortions in the signals shape or frequency response. In other words, previous ALBOs do not provide for varying degrees of attenuation in the fundamental and the different harmonics that make up a signal wave. Also, the previous ALBOs do not compensate for the loss of bandwidth in longer cables. These deficiencies prevent the use of the correct amount of distortion at very long cable lengths before outputting the signal to an equalization amplifier.